Thermal Bridging on Cathedral Ceiling
Climate zone 7B. 58lb snow load. No AC in most residential structures in this area.
I have a section of a proposed build that is 20×40 and the entire ceiling is cathedral with an 8/12 pitch, metal roof, and a couple of valley connections to the other rectangles of the floor plan. There will be heavy timber trusses in half and rafters and ridge beam in the other half. These are structural and also important architecturally. On top of that are concealed purlins (running perpendicular to the rafters and slope of the roof). These are planned to be 2x8s 24” oc. We use 5/8 OSB For roof decking in this area. I was planning to use a synthetic roof felt with a high perm rating like GAF Deck Armor. Then, thanks to learning things from this site, we’ll do the cross hatch 1×4 grid 24” oc and then a metal roof. We’ll be the first in the area (that I know of) not to put the metal right on top of Grace Triflex or something like it.
Seems like the best idea for insulation is flash and batt. We’ll use 5-6” of closed cell foam (CO banned the HFC blowing agent a year or two ago so that eases my conscience). I keep trying to figure out how to use repurposed roofing polyiso to counter the thermal bridging in the purlins, but this is somewhat problematic because I’ll need to add strapping outboard of the polyiso so the sheetrock screws don’t pop, thereby coming down the timber further with insulation, strapping, and sheetrock. Also, this makes hitting the ratio of closed cell foam more difficult.
what would seem to be easier is a sheet of EPS or GPS between the purlins and OSB. I’d go with the higher PSI rating. I like it because you put the EPS at the coldest section of the roof where it will perform the best, the foam is the air barrier, we’ll be sure to observe the 61% ratio of foam to fluff that Joe calls out.
the main question/problem is does this have a meaningful impact on the roof decking and purlin diaphragm? I know the conventional wisdom for rigid foam between studs and wall sheathing is it will reduce the shear strength. Though, it seems like Huber with Zip R has figured it works even with 2.5” of iso between the stud and sheathing.
building inspector is easy going and legitimately might not even see it so that’s not my issue. I don’t have an engineer for the house so it’s not an easy ask, e.g. I’m coming to GBA.
in general, does this sound like it will work and if so, how thick of foam board should I use? I assume for structure the thinner the better and otherwise the thicker the better. Plus, I don’t want to go too thick because I need to get the strapping for the vented roof attached through 5/8 OSB and foam to stud. That’s why I thought high PSI GPS and maybe 1” or 1.5”?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part